This study focuses on evaluating biological questions relevant to in vivo gene transfer with replication deficient recombinant adenovirus vectors using as a clinical model the thrombocytopenia associated with high dose chemotherapy for stage IV breast carcinoma. The therapeutic gene to be delivered is the human thrombpoetin cDNA, a hematologic cytokine that serves as a growth factor for megakaryocyte progenitors in bone marrow.